A liquid crystal display element makes use of optical anisotropy and dielectric anisotropy of a liquid crystal crystalline compound and is used for watches, electric calculators, various measuring instruments, panels for automobiles, word processors, electronic notebooks, printers, personal computers, televisions and portable telephones. There has been an increasing demand therefor year by year. A liquid crystal phase is between a solid phase and a liquid phase, and divided roughly into a nematic phase, a smectic phase and a cholesteric phase. Display elements making use of a nematic phase are most widely used among them at present. A lot of display modes have so far been developed, and a dynamic scattering mode (DS mode), a guest-host mode (GH mode), a twisted nematic mode (TN mode), a super twisted nematic mode (STN mode), a thin film transistor mode (TFT mode), a ferroelectric liquid crystal mode (FLC mode), etc. are known.
A recent development in these fields has mainly been in miniaturization, reduction in power consumption and increase in a response speed of liquid crystal display elements as seen in portable telephones, and liquid crystalline compounds and liquid crystal compositions have been required to have a low threshold voltage and a low viscosity.
A threshold voltage (Vth) is a function of a dielectric anisotropy (Δ∈) as shown by the following equation (Mol. Cryst. Liq. Cryst., 12, 57 (1970)):Vth=π(K/∈0Δ∈)1/2 wherein K is an elastic constant, and ∈0 is a dielectric constant in vacuo.
As seen from the above equation, increasing Δ∈ or decreasing K can be considered as a method for reducing Vth. However, it is still difficult to actually control K by conventional techniques, and a liquid crystal material having large Δ∈ has generally been used to meet the requirement. Therefore, liquid crystalline compounds having large Δ∈ have actively been developed. Viscosity is an element which affects a response speed of a liquid crystal molecule against an electric field, and a liquid crystalline compound having a low viscosity is preferably used in a large amount in order to prepare a liquid crystal composition showing a high speed response.
In recent years, liquid crystal display elements have become widely used in information terminals and portable games. These display elements are inevitably driven by batteries, and therefore, it is requested that they are driven at a low threshold voltage and have a low power consumption so as to be used for long time. Particularly in order to reduce a power consumption of an element itself, a reflective display element not requiring backlight has actively been developed recently. Liquid crystal compositions used for these reflective display elements are required to have a small refractive anisotropy as well as a low threshold voltage. Accordingly, it is important in this field to develop a liquid crystalline compound having a large dielectric anisotropy and a small refractive anisotropy as a liquid crystal material constituting the composition. The following compounds (13) and (14) (JP-A 2-233626) can be shown as a representative liquid crystal material for driving a display element at a low voltage, which is used for a liquid crystal display element of a TFT mode: wherein R represents an alkyl group.
Both the compounds (13) and (14) have a 3,4,5-trifluorophenyl group at the terminal of a molecule and are expected as a liquid crystal material for driving a display element at a low voltage. However, the compound (13) has a small dielectric anisotropy (Δ∈=about 10) for use in the reflective display element described above, and the compound (14) has a satisfactory dielectric anisotropy (Δ∈=about 12) but has a large refractive anisotropy of about 0.12, so that it is considered difficult to prepare a liquid crystal composition which can sufficiently satisfy the above requirements by using these compounds.
JP-A 10-204016 discloses the following compound (15) having a difluoromethyleneoxy group as a bonding group, which shows far larger dielectric anisotropy (Δ∈) of about 14 than that of the above compound (13) while showing a clearing point, a refractive anisotropy and a viscosity equivalent to those of the compound (13), and which is expected as a liquid crystal material for driving at a low voltage in various TFT modes and also as a liquid crystal material for the reflective display elements described above. In Formula, R represents an alkyl group.
JP-A 10-204016 discloses a process for producing the above compound having a difluoromethyleneoxy group as a bonding group, in which a corresponding ester derivative is converted to a thioester derivative with a Lawesson's reagent (Fieser 13, 38) and the derivative is further fluorinated by reacting with HF-pyridine in the presence of an oxidizing agent according to a method disclosed in JP-A 5-255165 to produce the above compound. In Formula, R1′ is an alkyl group; rings A1′, A2′ and A4′ are a 1,4-cyclohexylene group or a 1,4-phenylene group; and Z1′ is a single bond or —CH2CH2—.